Thallium system compositions and phases and methods of preparation are well documented in the literature. Thallium system superconductor compositions and a method of preparation were reported in "Bulk Superconductivity at 120 K in the Tl-Ca/Ba-Cu-O System," Z. Z. Sheng and A. M. Hermann, Nature Vol. 332, Mar. 10, 1988, pp. 138-139. Briefly described, appropriate amounts of powdered Tl.sub.2 O.sub.3, CaO and BaCu.sub.3 O.sub.4 to form a desired composition were completely mixed, ground and pressed into pellets. A tube furnace was heated to 880.degree. to 910.degree. C., and the pellets were placed in the furnace for three to five minutes with a flowing oxygen atmosphere. The pellets were removed from the furnace after heating, and quenched to room temperature in air, or furnace cooled to room temperature. The pellets were reaction sintered by the heat treatment, forming a superconducting oxide compound. Some quenched samples were subsequently annealed at 450.degree. C. in flowing oxygen for several hours.
An interesting feature of the thallium oxide superconductors, as well as a variety of other oxide superconductors, is that they are nonstoichiometric with respect to oxygen content. The oxygen content may vary. Though oxygen can vary over a small range, the variations can have an effect on the superconducting behavior of the compound. A convenient way of showing the composition of the oxide superconductors is by a formula showing the atomic ratio of each element, e.g., Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.10+y where y is a fraction of 1.
A number of the oxide compound compositions for the thallium system superconductors were reported in "Chemical Engineering and the Development of Hot Superconductors," R. Kumar, Chemical Engineering Progress, April 1990 pp. 17-27, and are shown by atomic ratio in Table 1 below. The critical temperature, T.sub.c, is shown for each composition.
TABLE I ______________________________________ Composition of Thallium System Superconductors by Atomic Ratio Tl Pb Ca Sr Ba Cu O T.sub.c ______________________________________ 2 -- -- -- 2 1 6 + y 80 2 -- 1 -- 2 2 8 + y 108 2 -- 2 -- 2 3 10 + y 125 2 -- 2 -- 3 4 12 + y 104 1 -- 1 -- 2 2 7 + y 80 1 -- 2 -- 2 3 9 + y 122 1 -- 2 -- 3 4 11 + y 120 0.5 0.5 1 2 -- 2 7 + y 90 0.5 0.5 2 2 -- 3 9 + y 122 1 -- -- 1 -- 1 5 + y 40 1 -- 0.5 2 -- 2 7 + y 90 ______________________________________
Since the time of the original disclosure of powder formed bulk thallium system superconducting compounds, thin films have been made by sequential thermal evaporation, and sequential electron beam evaporation. Both the sintered ceramic pellets and the thin films were treated by heating in flowing air or oxygen at temperatures of about 800.degree. to 900.degree. C. to form the superconducting compositions. However, thallium oxide has an appreciable vapor pressure at the temperatures required to form the superconductors. As a result, thallium can be vaporized during the 800.degree. to 900.degree. C. annealing treatments leading to loss of thallium from the superconductor.
In "Superconducting Thin Films of Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.y and Tl.sub.2 CaBa.sub.2 Cu.sub.2 O.sub.y," D. S. Ginley et al., Physica C 156, 1988, pp. 592-598, incorporated herein by reference, it was reported that thin films of the thallium system were annealed in the presence of thallium metal to suppress thallium loss from the superconductor. This method was found to be an unreproducible technique for forming the superconducting compositions. In another method, thin films were annealed while sandwiched between pellets of the thallium system superconductor consolidated from powders with the correct stoichiometry for the composition desired in the thin film. Thus, if a film of the superconducting composition Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.10+y was being produced, the bulk ceramic pellet composition was prepared to be Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.10+y.
A strong correlation was found between film quality and annealing the film in the presence of the bulk ceramic of the desired composition. Films having a thallium content slightly greater than the stoichiometric composition had improved current carrying capacity, while those that were thallium deficient had a lower current carrying capacity. High current carrying capacity in polycrystalline oxide superconductors is believed to be associated with strongly interconnected or linked grains in the polycrystalline material, and low current carrying capacity is believed to be associated with poorly connected or weak linked grains. The individual crystals in polycrystalline superconductors are sometimes herein referred to as grains.
The high transition temperatures of the thallium system superconductors makes commercial use in devices such as motors, magnets, and power generation and transmission equipment operating at liquid nitrogen temperature and above possible. However, to be of practical value a ceramic superconductor must be able to carry or conduct a substantial current, at least approaching the current carrying capacity of alloy superconductors such as Nb.sub.3 Sn. The high current carrying capacity must be evident at temperatures up to the critical temperature without quenching or loss of superconductivity as evidenced by a substantial increase in electrical resistance.
In some applications the high current carrying capacity must also be evident in the presence of magnetic fields. It is known that current carrying capacity can be greatly diminished in the presence of a magnetic field, so that the superconductor decreases in current carrying capacity as magnetic field increases up to a critical magnetic field above which the superconductor is quenched. A superconductor having high magnetic field behavior is characterized by a low reduction in current carrying capacity when the superconductor is in the presence of a magnetic field.
Another requirement for commercial application of ceramic superconductors is the development of reproducible and efficient methods for the fabrication of polycrystalline forms, particularly films of the thallium system compositions and most particularly films of the 1223 phase.
An object of this invention is a method of forming polycrystalline thallium system superconductors having improved current carrying capacity at operating temperatures and in magnetic fields.
Another object of this invention is to provide a method for making improved polycrystalline thallium system superconductor films by incorporating a small amount of silver in the precursor oxide film prior to the thallium-incorporating reaction which forms the thallium containing superconductor phase.